Petrography in the context of Outcrop

A stratigraphic unit is a volume of rock of identifiable origin and relative age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic, lithologic or paleontologic features (facies) that characterize it.

Units must be mappable and distinct from one another, but the contact need not be particularly distinct. For instance, a unit may be defined by terms such as "when the sandstone component exceeds 75%".

A petrographic microscope is a type of optical microscope used to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks. The method includes aspects of polarized light microscopy (PLM).

In optical mineralogy and petrography, a thin section (or petrographic thin section) is a thin slice of a rock or mineral sample, prepared in a laboratory, for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a diamond saw and ground optically flat. It is then mounted on a glass slide and then ground smooth using progressively finer abrasive grit until the sample is only 30 μm thick. The method uses the Michel-Lévy interference colour chart to determine thickness, typically using quartz as the thickness gauge because it is one of the most abundant minerals.

When placed between two polarizing filters set at right angles to each other, the optical properties of the minerals in the thin section alter the colour and intensity of the light as seen by the viewer. As different minerals have different optical properties, most rock-forming minerals can be easily identified. Plagioclase for example can be seen in the photo on the right as a clear mineral with multiple parallel twinning planes. The large blue-green minerals are clinopyroxene with some exsolution of orthopyroxene.

In the archaeology of the Stone Age, an industry or technocomplex is a typological classification of stone tools.

An industry consists of a number of lithic assemblages, typically including a range of different types of tools, that are grouped together on the basis of shared technological or morphological characteristics. For example, the Acheulean industry includes hand-axes, cleavers, scrapers and other tools with different forms, but which were all manufactured by the symmetrical reduction of a bifacial core producing large flakes. Industries are usually named after a type site where these characteristics were first observed (e.g. the Mousterian industry is named after the site of Le Moustier). By contrast, Neolithic axeheads from the Langdale axe industry were recognised as a type well before the centre at Great Langdale was identified by finds of debitage and other remains of the production, and confirmed by petrography (geological analysis). The stone was quarried and rough axe heads were produced there, to be more finely worked and polished elsewhere.

Igneous petrology is the study of igneous rocks—those that are formed from magma. As a branch of geology, igneous petrology is closely related to volcanology, tectonophysics, and petrology in general. The modern study of igneous rocks uses a number of techniques, some of them developed in the fields of chemistry, physics, or other earth sciences. Petrography, crystallography, and isotopic studies are common methods used in igneous petrology.

Stereology is a branch of applied mathematics that is the three-dimensional interpretation of two-dimensional cross sections of materials or tissues. It provides practical techniques for extracting quantitative information about a three-dimensional material from measurements made on two-dimensional planar sections of the material. Stereology is a method that utilizes random, systematic sampling to provide unbiased and quantitative data. It is an important and efficient tool in many applications of microscopy (such as petrography, materials science, and biosciences including histology, bone and neuroanatomy). Stereology is a developing science with many important innovations being developed mainly in Europe. New innovations such as the proportionator continue to make important improvements in the efficiency of stereological procedures.

In addition to two-dimensional plane sections, stereology also applies to three-dimensional slabs (e.g. 3D microscope images), one-dimensional probes (e.g. needle biopsy), projected images, and other kinds of 'sampling'. It is especially useful when the sample has a lower spatial dimension than the original material.Hence, stereology is often defined as the science of estimating higher-dimensional information from lower-dimensional samples.

Ceramography is the art and science of preparation, examination and evaluation of ceramic microstructures. Ceramography can be thought of as the metallography of ceramics. The microstructure is the structure level of approximately 0.1 to 100 μm, between the minimum wavelength of visible light and the resolution limit of the naked eye. The microstructure includes most grains, secondary phases, grain boundaries, pores, micro-cracks and hardness microindentations. Most bulk mechanical, optical, thermal, electrical and magnetic properties are significantly affected by the microstructure. The fabrication method and process conditions are generally indicated by the microstructure. The root cause of many ceramic failures is evident in the microstructure. Ceramography is part of the broader field of materialography, which includes all the microscopic techniques of material analysis, such as metallography, petrography and plastography. Ceramography is usually reserved for high-performance ceramics for industrial applications, such as 85–99.9% alumina (Al₂O₃) in Fig. 1, zirconia (ZrO₂), silicon carbide (SiC), silicon nitride (Si₃N₄), and ceramic-matrix composites. It is seldom used on whiteware ceramics such as sanitaryware, wall tiles and dishware.

• Ceramographic microstructures
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  Fig. 1: Thermally etched 99.9% alumina
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  Fig. 2: Thin section of 99.9% alumina

The stone quarries of ancient Egypt once produced quality stone for the building of tombs and temples and for decorative monuments such as sarcophagi, stelae, and statues. These quarries are now recognised archaeological sites. Ancient quarry sites in the Nile valley accounted for much of the limestone and sandstone used as building stone for temples, monuments, and pyramids. Eighty percent of the ancient sites are located in the Nile valley; some of them have disappeared under the waters of Lake Nasser and some others were lost due to modern mining activity.

Some of the sites are well identified and the chemical composition of their stones is also well known, allowing the geographical origin of most of the monuments to be traced using petrographic techniques, including neutron activation analysis.

Enstatite is a mineral; the magnesium endmember of the pyroxene silicate mineral series enstatite (MgSiO₃) – ferrosilite (FeSiO₃). The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and metamorphic rocks. The intermediate composition, (Mg,Fe)SiO
₃, has historically been known as hypersthene, although this name has been formally abandoned and replaced by orthopyroxene. When determined petrographically or chemically the composition is given as relative proportions of enstatite (En) and ferrosilite (Fs) (e.g., En₈₀Fs₂₀).

Amanz Gressly (17 July 1814 – 13 April 1865) was a Swiss geologist and paleontologist. He introduced the use of the term facies in geology, and is considered one of the founders of modern stratigraphy and paleoecology.

He initially studied medicine at Strasbourg, but his interest subsequently switched to geology, and from 1836 onward, he worked as an assistant to Louis Agassiz. In 1838 he published Observations géologiques sur le Jura Soleurois (Geological observations involving the Solothurn Jura), in which he introduced the "concept of facies" to describe the environments and conditions of the origin of sedimentary rocks based on their petrographic attributes and fossil affiliations.